On free will-6: The 1963 Grey Walter experiment

(For previous posts in this series, see here.)

In the previous post, I provided a schematic description of two models of how the brain works, one with free will and the other without it. The traditional brain model with free will is given by

(D)                                    GES
will → conscious thoughts → unconscious neural activity → action

Our genes (G), environment (E), and the inherent randomness in the laws of nature (S) all contribute right up to the present instant to the brain’s structure and unconscious neural activity. But in this model, there is a separate branch in which our (uncaused) free will makes decisions first which manifests itself as a conscious thought. In this model there should be a definite temporal sequence in which the act of will occurs first, followed by conscious thoughts, then unconscious brain activity caused by that conscious thought, and finally the action.

The model without free will is given by

(G)                                GES
conscious thoughts/will ↔ unconscious neural activity → action

In this model, since conscious thoughts and what we think of as free will are not prior to unconscious neural activity but instead are the products of it, they need not temporally precede it.

The way that researchers investigate whether the idea of free will is tenable is by looking at the time sequence of events. One of the earliest experiments that threw the traditional model of (D) into doubt was done by neurosurgeon W. Grey Walter in 1963. Daniel Dennett, (Consciousness Explained, 1991, p. 167) describes the experiment.

Grey Walter performed his experiment with patients in whose motor cortex he had implanted electrodes. He wanted to test the hypothesis that certain bursts of recorded activity were the initiators of intentional actions. So he arranged for each patient to look at slides from a carousel projector. The patient could advance the carousel at will, by pressing the button on the controller. (Note the similarity to Libet’s experiment: This was a “free” decision, timed only by an endogenous rise in boredom, or curiosity about the next slide, or distraction, or whatever.) Unbeknownst to the patient, however, the controller button was a dummy, not attached to the slide projector at all! What actually advanced the slides was the amplified signal from the electrode implanted in the patient’s motor cortex. (My italics)

As far as the patient was concerned, and according to the model (D) that has free will, the temporal sequence the patients expect should be conscious thought → button push → slide advance. But the direct measurement of motor cortex brain activity introduces a new time step that is unknown to the patient but can be measured by the researchers. As a result, if free will exists, the patient should first become aware of making a decision, then send a command to the motor cortex, which produces both the amplified signal (which causes the slide to advance) and sends a signal to the finger to push the button. If the slide advanced after the patient was conscious of making a decision to push the button but before the button was actually pushed, that would definitely puzzle the patients because they were under the impression that it was their pushing of the button that advanced the slide. But all it would really imply to the researchers is that the speed with which the motor neuron activity sends an electrical signal to the slide projector is greater than the speed with which the motor neuron sends the push signal to the finger.

So what happened? Dennett continues the story:

One might suppose that the patients would notice nothing out of the ordinary, but in fact they were startled by the effect, because it seemed to them as if the slide projector was anticipating their decisions. They reported that just as they were “about to” push the button, but before they had actually decided to do so, the projector would advance the slide – and they would find themselves pressing the button with the worry that it was going to advance the slide twice! (My italics)

In other words, the motor cortex activity that triggered the slide advance seemed to occur not only before the finger pushed the button but even before the patients said they were conscious of making the decision to push the button. This experiment could be interpreted as an early indication that there was a spike in brain activity about half a second before the person was conscious of making a decision to carry out an action.

At first glance, this experimental result might seem to be a devastating blow to the idea of free will. If the brain’s unconscious neural activity makes and executes a decision before a person is conscious of making that same decision, then that refutes the expected temporal sequence that is at the heart of the model (D) that has free will in it.

But we have to be careful of jumping to that conclusion. There is a danger of over-interpreting these results because the experimenter is dependent on the patients’ reporting of when they had the conscious thought and Dennett argues that the brain is not a reliable source of information about its own workings, for reasons to be outlined in the next post in this series.

Next: How reliable a historian is the brain?


  1. says

    Hey Mano,

    Have you ever read a book called psychocybernetics by a guy called maxwell waltz?

    Its quite an interesting insight into the importance of subconscious and ties into concepts you’re discussing here.

    I’d be interested to hear your take on it in light of this post.


  2. says

    Similarly, it amazes me how much we are influenced by other people.

    Have you seen the clip of the woman in a room with the smoke? Smoke starts to fill the room she’s in. The other people are “in” on the experiment and don’t leave the room. Because they stay, the woman stays too.

    We’d all say that of course we’d leave the room, but we probably wouldn’t.

    The clip should be on YouTube if you want to check it out.

Leave a Reply

Your email address will not be published. Required fields are marked *